Many circuits and systems include the need for protection against inrush current. By "inrush current" is meant an initial high current flow, usually a short duration surge, which is usually attributable to a highly reactive initial power load. Inrush current is particularly a problem within telecommunications systems which typically provide distributed 48 volt battery to all of the circuits throughout the network.
Telecommunications circuits are usually formed as small plug-in circuit boards carrying and interconnecting solid state and passive circuit elements. Such circuit elements typically require voltages lower than the nominal 48 volt common battery, and so DC to DC power conversion modules are typically included on each such plug-in board to provide suitable lower operating DC voltages, such as 5 volts and 12 volts. One characteristic of the ubiquitous DC to DC power conversion module is that it presents a highly capacitive initial power load to the supply bus, with resultant enormous instantaneous current flow when power is first applied.
When the module is plugged into a backplane supplying the 48 volt common battery, unless limited, the inrush current has been measured at over 40 Amperes (100 microsecond duration) with a 48 volt, 15 watt DC to DC conversion supply. This inrush current is well over the rating of typical connector pins which are usually rated at about one to two Amperes maximum. The inrush current also may cause sparking with resultant corrosion of the connector pins and sockets and other exposed elements, due to the ozone thus created. Input voltage regulation to other on-line circuit boards within the system may be adversely affected during the interval of a high inrush current surge.
Particularly within telecommunications systems, where the availability of continuous service for the service subscriber is mandatory, modules are plugged into and out of the system without shutting down the 48 volt common battery. Intermittant contact of power pins on insertion and removal of circuit boards requires that any inrush current limiting circuit respond very quickly. It is virtually impossible to insert or remove a circuit board with a 96 pin DIN connector, for example, without some intermittant contact of pins carrying the 48 volt common battery primary power supply.
One prior approach to inrush current limiting is a simple series resistor. Such a resistor must be about 10 Ohms to limit inrush current to 5 Amperes with a 48 volt common battery supply. With a nominal steady state input current of 0.65 Amperes, use of a 10 Ohm limiting resistor results in a series voltage drop of 6.5 volts, which will reduce the input voltage range of the circuit board and which will result in over four watts heat dissipation at the limiting resistor.
A relay may be used to short out a series limiting resistor after the inrush current surge has passed. This approach effectively reduces the series voltage drop during normal operation of the circuit board elements. However, the slow mechanical time constant of electromechanical relays allows high currents to flow while the relay contacts remain closed when contact to the power pins is intermittant on insertion or removal, which is almost unavoidable.
An active circuit, in lieu of an electromechanical relay, to short out a series resistor is another approach that may be followed. However, unlike the circuit of the present invention, such approach still requires a series current limiting resistor which adds to the cost of the protection circuit and which creates a time constant. A circuit having a time constant characteristic will cause problems under intermittant contact conditions.
Negative temperature coefficient (NTC) resistors may be used to limit inrush current. At room temperature, these NTC resistors may have resistances of up to 100 Ohms, which reduce to a few ohms at high temperature. Upon insertion of the circuit board, inrush current is reduced. As current continues to be drawn through the NTC resistor, it warms by as much as 20 to 30 degrees Centigrade, resulting in lower resistance and voltage drop. The thermal time constant in NTC resistors allows high inrush currents to flow on intermittant contact on circuit board removal, or on a reinsertion thereof just after removal. In addition, NTC resistors are not a good solution for a system that must operate over a temperature range wider than 20 degrees Centigrade, as the resistance thereof may drop at high temperature, allowing high inrush current to flow. The resistance of NTC resistors (and series voltage drop) may also be too high when the ambient temperature is very low after warmup.
Another approach, quite similar to the present invention, is acknowledged in the prior art. For example, U.S. Pat. No. 4,494,064 (column 1 line 66 through column 2, line 8) characterizes another prior art approach as employing a series connected linear limiter circuit utilizing junction transistors or field effect transistors for protection against inrush current. According to that disclosure, use of active devices in series for control of inrush current surges from a DC bus is fast and simple, but active devices exhibit excessive power dissipation during current limiting and are said to be suitable only for low current applications. While that disclosure recognizes a possible solution to the problem, no circuit details or design considerations are provided, and the worker in the art is left to his skill and speculation.
Thus, a hitherto unsolved need has remained for an effective inrush current limiting circuit which overcomes the limitations and drawbacks of the prior approaches.